Communications networks sometimes experience network failures. For example, either a network node, such as a digital cross-connect switch (DXC), may fail or a trunk, like a fiber optic cable, that connects two nodes may fail. Such network failures may result from a variety of circumstances, including mechanical failure, operator error, or environmental conditions, like an earthquake, which may render a network component inoperable.
To quickly reroute the network traffic around a network failure, network restoration systems have been developed. These systems generate restoration plans ("plans") which contain instructions for rerouting network traffic around a network failure. This rerouting around a network failure is sometimes referred to as "restoring the network." By preplanning network restoration, the time necessary for rerouting network traffic is greatly reduced, which lessens the negative impact of a network failure on network traffic. For example, FIG. 1 depicts a network 100 comprising nodes 102-106 and trunks 108-112, which interconnect the nodes. With respect to the network 100, a plan for the failure of trunk 112, which prevents the direct transfer of traffic from node A to node C, may be as follows: (1) disconnect node A and node C, (2) connect node A with node B such that traffic may be transferred over trunk 108, and (3) connect node B with node C such that traffic may be transferred over trunk 110. "Connecting two nodes" refers to either automatically or manually indicating to the two nodes that they may communicate with each other over the trunk that interconnects the nodes. After two nodes have received such an indication, each node expects to receive data from the other, and each node is prepared to negotiate to exchange data using a predesignated protocol. When two nodes are disconnected either manually or automatically, the nodes no longer attempt to exchange data over the trunk that interconnects the nodes. Once this example plan has been implemented, network traffic will circumvent the failed trunk 112 and will instead be routed between nodes A and node C via node B, so that devices connected to node A can communicate with devices connected to node B.
Conventional network restoration systems create plans to prepare for a trunk failure, a node failure, or a network failure that will affect a number of trunks and nodes. These network restoration systems create plans for a particular failure (e.g., a trunk) by first examining the available spare network segments that may be used to route traffic around the failed trunk. A network segment is a path in the network formed by one or more contiguous trunks and the nodes that interconnect those trunks. Many networks, such as the public telephone network, have two types of network segments: a traffic segment, which is a network segment used for traffic during normal operations of the network, and a spare segment, which is a segment not typically used for network traffic during normal operations of the network, but which is instead used in the event of a network failure to circumvent traffic around the network component that failed. After examining the available spare network segments, the network restoration system selects the best spare segment based on a number of factors, including which spare segment spans the shortest distance, which spare segment has the greatest bandwidth, and which spare segment has the least amount of network traffic.
Conventional network restoration systems continually update the plans for a network to ensure that the plans are current. That is, for each node and trunk in the network, the network restoration system generates a plan for that network component, and when it is completed with all of the network components, the network restoration system immediately starts over again. Although in a small network this continual regeneration may be acceptable, for a large network, like the public telephone network, there may be between 800 and 1500 plans in existence at any given time, which may require up to three days to regenerate. During this three-day time frame, the topology of the network may change, and a number of the plans may become ineffective to reroute traffic around a network failure because, for example, the topology change may have removed a network component which was part of the plan. Additionally, this continuous update of the plans places an enormous burden on the resources of the network restoration system. It is therefore desirable to improve network restoration systems.